Seismic events often cause dynamic responses in structures sufficient to permanently damage or destroy the primary load-bearing members. Extensive research into the dynamic response of building structures has revealed that modest applications of ancillary damping can dramatically reduce deflections and stresses due to seismic excitation. This ancillary damping may be provided by either yielding and hysteretic energy dissipation in primary structural elements or the inclusion of devices specifically designed to absorb energy while remaining within the elastic range of the primary structure. These latter devices offer the great advantage of minimizing damage to curtain walls, interior structures, and other building systems. In some cases these auxiliary dampers are sacrificial and need replacement after extreme events, while in other cases they may sustain many extreme load cycles without significant maintenance. If effective ancillary damping mechanisms can be developed, in retrofit applications, for multi-storied steel frame buildings, then seismic upgrades of numerous buildings can be significantly expedited.
A wide variety of passive damping schemes have been marketed and implemented with varying degrees of success. These damping devices assume many forms characterized by a wide range of complexity and cost as outlined below; friction dampers, hysteretic (yielding) dampers, lead extrusion dampers, shape memory alloy devices, viscoelastic (rubber or rubber/metal hybrid) isolators, magnetostrictive or magnetorheological devices, tuned mass dampers, and tuned liquid/liquid column dampers. Aside from the tuned mass and/or liquid dampers, the basic damper configuration typically spans a building frame bay, either via a diagonal strut or a chevron brace arrangement. The key design parameters for any of the damper types include maximum force capacity and damper stroke (peak-to-peak in a load cycle). The different damper technologies exhibit hysteresis curves, bounded by these load and stroke parameters, whose shape depends upon the physical characteristics of the damper, and, in the case of viscous dampers, the velocity of the building motion. The required damper stroke is determined by the building displacement limits set either by the appropriate building code or by the builder""s assessment of the acceptable damage threshold. In the US, building shear displacement angles (measured as the horizontal displacement of an upper story: the height between the upper story and the story beneath it) of 1:200 are generally considered to be limiting cases, while in Japan, shear displacement angles of 1:100 are tolerated.
One successful example of the damping devices outlined above has been the line of fluid viscous dampers by Taylor Devices, Inc. of North Tonawanda, N.Y. These fluid viscous dampers are essentially superscale versions of automotive shock absorbers, with load capacities ranging from 10 kips to 2000 kips, and strokes of up to 120 inches. While providing effective damping forces out of phase with the excitation, the fluid viscous dampers are relatively complex and costly and may not provide the desired design flexibility and longevity.
A recent development in hysteretic dampers fabricated from low strength steel and concrete by Nippon Steel has shown good performance with a minimum of complexity and cost. This damper mechanism has been used in several new-build projects in Japan. One implementation of this damper brace is a welded steel box of approximately 55 cm by 65 cm filled with concrete enclosing a low strength steel brace having a cruciform shape. Braces have been fabricated having a free length of just over 20 meters and weighing approximately 34 tons. The weight of the concrete-filled steel sleeve is very high and renders retrofit application of the damping brace difficult, if not impossible. The cost of this damping method is driven upward by the proprietary nature of the very low yield strength steel (100 Mpa/14.5 ksi) used in the strut.
The technology options for seismic energy absorbers currently available include: the Nippon Steel hysteretic strut brace and sleeve combination, yielding plate dampers, and viscous dampers such as the Taylor Devices line. While there are several other technologies that have some promise (lead extrusion, shape memory alloy, magnetorestrictive), these are not currently available on a commodity basis.
A lightweight hysteretic damper is useful for framed buildings to reduce seismic response levels. A seismic brace incorporates a low-strength aluminum multi-armed strut that plastically deforms during a seismic event, damping a building""s response because of the hysteresis in the strut material stress-strain curve. This strut is surrounded by a collar providing high bending stiffness, but no extensional stiffness, to prevent a low energy buckling failure of the brace in compression. The collar is composed of an outer sleeve of composite materials or metal construction, and spacers to provide the requisite load transfer from the strut which is free floating within the collar. Substantial improvements in weight-specific energy absorption and cost as compared to extant damper concepts are possible.
The hysteretic seismic damper employing a yielding central strut surrounded by a buckling suppression collar is utilized mounted along one or more diagonals of a building frame, and reduces structure seismic response by absorbing strain energy (providing extra damping). In order to maximize this damping energy absorption, the brace remains stable in both tension and compression load cycles to a significant level of plastic strain. When under significant compressive strain, the tangent modulus of the structural material is much lower than its initial modulus, introducing the requirement for a very rigid collar to prevent strut/brace buckling. Composite materials provide an opportunity to create such a collar at minimum weight and cost while metals employ known manufacturing methods.
In one embodiment utilizing a cruciform strut, the aluminum strut is surrounded by four hollow quarter-rounds of metal or composite construction, each of which contains both longitudinal and shear stiffness that in the aggregate is sufficient to prevent strut buckling up to its compressive yield strength. The quarter rounds are attached to one another and contained about the aluminum strut by a sleeve providing reinforcement mostly in the hoop/bias direction. For field assembly, the collar is assembled around the strut with the sleeve bonded to the spacers in the field using a room-temperature-curing adhesive. Factory assembly is an alternative although this field assembly embodiment is particularly well suited to retrofit applications.
In another embodiment, the aluminum strut is surrounded by four lightweight quarter-rounds, each of which is sufficient to transfer radial stresses to an outer sleeve. The four quarter-rounds may be attached to one another and are contained about the aluminum strut by a reinforced sleeve that contains both longitudinal and shear stiffness sufficient to prevent strut buckling up to its compressive yield strength. The sleeve is bonded to the spacers with an adhesive. This concept is optimized for initial installation applications since it can be constructed at greater lengths than the previous embodiment. Other aspects, features, and advantages of the present invention are disclosed in the detailed description that follows.